Dietpi/Plex DAC sstting help please


Installed dietpi/plex on a pi 4 recently mainly to play audio files. Can play audio on laptop/tablet thro’ speaker/headphones OK.
My intention is to send files to the DAC (Schiit bifrost) connected with USB then to the amp/speakers.

I couldn’t find the setting in Plex or dietpi to use the USB DAC, tried dietpi-config audio;

auto detection
USB Audio DAC, and
Device USB Audio.
Neither of those settings work, unfortunately.

Can someone help regarding the DAC output setting please.